We have known for decades that if heart rate during an all-aerobic (below lactate/anaerobic threshold) workout rises while the intensity (power or pace) stays the same then the athlete is not operating efficiently and his or her aerobic endurance is questionable. The same is true if heart rate stays the same and power decreases or the pace slows.

Variability Index is a way to measure how smooth or “variable” your power output was during the ride. It is calculated by dividing your Normalized Power by your Average Power. A steady and even output, like during a triathlon, should have a VI of 1.05 or less. During a criterium race your VI may be as high as 1.2 or more. Learn more about this key metric…